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Osmotic Action | ||
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| We Solve Moisture Vapor and Alkalinity Problems |
VAP I® 2000 and VAP I® pH |
| Osmotic Action | |||||
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A recent investigation
into the causes and effects of moisture contained in the concrete
substrate has revealed some interesting explanations. This investigation was conducted by the very prestigious "Polymer Institute, Dr. R. Stenner G.m.b.H.," Germany. It covers coating failures due to osmotic action only. Hydrostatic pressure and capillary action are not part of this analysis because most coating - adhesion failures are clearly attributable to osmotic action. The following is a summary of this investigation: Osmosis is defined as "the passage of a pure solvent into a solution through a semi-permeable membrane." It is the result of different concentrations of soluble compounds on each side of a semi-permeable membrane due to water/moisture presence. Typical for this action is that it usually takes 3 weeks to 6 months to delaminate coatings without surface/substrate residue adhering to the coating adhesion side. The time frame within this delamination takes place depends on the nature of the coating (composition) as well as that of the concrete. The following is required to create osmotic action:
Osmotic action can be due to moisture penetrating concrete from underneath (slab on grade for example) or because of residual moisture within the concrete substrate itself. This moisture penetrates due to osmotic action to the underside (adhesion contact area) of coatings/membranes and creates a highly alkaline (basic) environment. Primarily responsible for this alkalinity are highly soluble metal oxides usually contained in cement in various concentrations like potassium and sodium hydroxide. Calcium hydroxide is also present in this solution but contributes to the alkalinity to a lesser extent. Given this alkaline environment with a pH of 13-14, any material that comes in contact with it has to be totally unaffected by it long term to prevent chemical attack with subsequent debonding from the substrate. Considering this, one has to avoid using material systems containing alkaline sensitive compounds that can leach out and greatly contribute to osmotic action by adding to the solution that causes osmosis underneath coatings. Analysis of water build up underneath coatings due to osmosis revealed solutions containing organic substances like solvents, plastizisers, non- reacted resins and amine compounds, etc. that leached out of these products. In order to reduce osmotic action and fully resist this damaging condition the following material properties, amongst others, are an absolute requirement:
In most cases water pressure does not cause adhesion failures due to "osmotic pressure" as often suspected. Even under extreme conditions rarely found in construction (100% relative humidity with 100° F temperatures) the pressure would not reach 1 psi. Responsible for the delamination of coatings/membranes due to osmotic action is the chemical composition of these materials. This explains why coating/membranes can debond completely or partially without any or very little surface residue adhering to the adhesion side, even though the initial adhesion after application was strong. |
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